The invention relates to an apparatus for determining the respiratory passageway impedance comprising a breathing tube which exhibits a pulse generator for subjecting the breath flow with flow pulsations of a higher frequency as compared with the respiratory frequency, and which exhibits a selective pressure measuring apparatus for the correspondingly higher-frequency pressure fluctuations, and whose tube-end not facing the mouthpiece is terminated with a flow resistance having a known resistance value; for example, a hose, as the comparison resistance, such that a first partial current of the higher frequency total current flows over the flow resistance, whereas a second partial current flows via the mouthpiece into the respiratory tract of an examination subject.
An apparatus of the type initially cited is known from the U.S. Pat. No. 4,051,843. With an apparatus of this type, the respiratory passageway resistance can be indicated rapidly and in a problem-free manner as a function of the impressed flow, the complex comparison flow impedance having a known impedance value, and the measured pressure valve. In a further development of an apparatus of this type, means were provided in the measuring apparatus for the simultaneous detection of the phase angle of the pressure measured value and means for representing, or converting, respectively, the measured value with the object of determining the magnitude and phase of the complex respiratory passageway impedance. However, the equations derived for this purpose show that the connection between the measured values and the magnitudes to be determined is relatively complicated. In said letters patent, the complex respiratory passageway impedance must first be determined from diagrams with curved coordinates, or by means of programmable computers, respectively.